1. Field of the Invention
The present invention relates to a capacitive detection type electro-mechanical transducer, and specifically relates to a capacitive detection type electro-mechanical transducer that can be employed for, e.g., a capacitive detection type ultrasonic sensor.
2. Description of the Related Art
There are known diagnostic apparatuses for breast tumors, which apply the photoacoustic effect.
For example, in order to detect ultrasonic waves from an object, a PVDF sensor (approximately 75 mm square) including a total of 590 pixels arranged with a pitch of approximately 3 mm, each pixel having a width of 2 mm is used. PVDF sensors have a wider bandwidth compared to PZT sensors.
As with PVDF sensors, capacitive detection type ultrasonic sensors (CMUTs: Capacitive Micromachined Ultrasonic Transducers) using a MEMS technique have been proposed as sensors having a wider bandwidth compared to PZT sensors (A. S. Ergun, Y. Huang, X. Zhuang, O. Oralkan, G. G. Yaralioglu, and B. T. Khuri-Yakub, “Capacitive micromachined ultrasonic transducers: fabrication technology”, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 52, No. 12, p.p. 2242-2258, December 2005).
CMUTs can be formed at low cost because they are formed by applying a semiconductor process. For CMUTs, a pixel pitch of 250 to 400 μm is used depending on the frequency band of the object to be measured.
It can be expected that use of a CMUT in a breast tumor diagnostic apparatus applying the photoacoustic effect brings cost reduction in the sensor part of the apparatus.
However, as a result of diligent study, the present inventors found that if the pixel size of a CMUT is simply increased (i.e., the number of cells (or elements) included in one pixel is increased), the following problems arise.
A CMUT, which is an electrostatic capacitive detection type sensor, includes a detection circuit that detects a current change caused by a capacitance change.
If a pixel size employed for a conventional CMUT is increased to a size for a breast tumor diagnostic apparatus, a larger capacitance load will be put on each pixel in the CMUT. Thus, a larger load will be put on each detection circuit, causing a problem in requiring the detection bandwidth to be narrowed in order for the detection circuit to stably operate.
In view of the above problem, an object of the present invention is to provide a capacitive detection type electro-mechanical transducer enabling expansion of a frequency bandwidth for each detection circuit to stably operate, and thus, enabling provision of a wide detection bandwidth even where the size of each pixel is increased.